Build the NestJS AuthGuard Turning verification into a reusable request gate
Course principle: a user ID becomes trusted only after the access token has been cryptographically verified by the backend.
Authenticated identity type // common/types/authenticated-user.ts export type AuthenticatedUser = { id: string email?: string role?: string sessionId?: string }
Guard implementation // auth/supabase-auth.guard.ts import { CanActivate, ExecutionContext, Injectable, UnauthorizedException, } from '@nestjs/common' import type { FastifyRequest } from 'fastify' import { SupabaseAuthService } from './supabase-auth.service'
type AuthenticatedRequest = FastifyRequest & { user?: { id: string email?: string role?: string sessionId?: string } }
@Injectable() export class SupabaseAuthGuard implements CanActivate { constructor( private readonly auth: SupabaseAuthService, ) {}
async canActivate( context: ExecutionContext, ): Promise<boolean> { const request = context.switchToHttp() .getRequest<AuthenticatedRequest>()
const token = this.extractBearerToken(request)
const claims = await this.auth.verifyAccessToken(token)request.user = { id: claims.sub, email: typeof claims.email === 'string' ? claims.email : undefined, role: typeof claims.role === 'string' ? claims.role : undefined, sessionId: typeof claims.session_id === 'string' ? claims.session_id : undefined, }
return true }
private extractBearerToken( request: FastifyRequest, ): string { const value = request.headers.authorization const [scheme, token] = value?.split(' ') ?? []
if (scheme !== 'Bearer' || !token) { throw new UnauthorizedException( 'Bearer access token required', ) }
return token
}
}What the guard does not do • It does not decide Workspace permissions.
• It does not decide Task ownership.
• It does not decide whether a workflow transition is allowed.
• It does not query Prisma merely to prove the JWT signature.
Authentication creates trusted identity. Authorization and business rules remain separate layers.
Source note: verified against current Supabase Auth/getClaims/signing-key guidance and NestJS authentication/authorization docs. Full URLs and version notes are included in SOURCE_NOTES.txt.
Build the NestJS AuthGuard matters because it changes a real boundary in TaskFlow. The goal is to understand the inputs, owner, observable output, and failure modes before adding more abstraction.
Restate the goal of “Build the NestJS AuthGuard” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: route data → UI state → component tree → interaction → accessible feedback. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Authentication guard + verification service + current-user helper + protected /auth/me and task routes.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
if (query.isPending) return <TaskSkeleton />;
if (query.isError) return <TaskError retry={query.refetch} />;
if (!query.data.length) return <EmptyTasks />;
return <TaskTable tasks={query.data} />;Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.
Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.
Without copying the final source, create a second small scenario that applies “Build the NestJS AuthGuard” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Normalize verified claims into an application user identity object.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Logging bearer tokens during debugging
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.